Implantable pressure sensor

ABSTRACT

A device for use in pressure sensing such as hydrocephalus shunts, having a housing enclosing a chamber with at least one port communicating with the chamber. A wall of the chamber includes a flexible portion or thin diaphragm that deflects with transitions in pressure to contact an upstand structure of the housing. A pressure sensor is contained within an enclosure sealed from the chamber by a flexible membrane and which receives fluid pressure from the chamber. This arrangement allows for calibration of the device by identifying a knee feature in pressure data associated with the diaphragm making contact with the upstand.

FIELD OF THE INVENTION

The present invention relates to pressure sensors, particularly topressure sensors for implanting on a long term basis.

SUMMARY OF THE PRIOR ART

Implanted pressure sensors are known for use as a stand alone device, oras part of a larger device. One particular issue with implanted sensorsis to maintain an accurate output. Sensors typically are either relativepressure sensors, or absolute pressure sensors. Relative pressuresensors work with a reference pressure that can be applied from outsidethe body. Absolute pressure sensors are self-contained, and include asealed reference pressure chamber. Usually the absolute pressure sensorreference chamber is evacuated, to minimize the impact of variation intemperature, but another known reference pressure may be provided whereeither the temperature will be stable in use, or a temperature sensor isincluded in the system which can be used to adjust pressure readingsfrom the sensor.

A relative pressure sensor has a pathway for externally applying areference pressure. This is usually a catheter or tube connecting to thesensor and extending from the body of the user. This makes the sensorunsuitable for long term use, or use outside the clinical environment.

Absolute pressure sensors are known to have problems with the sensoroutput drifting over time. They are susceptible to gradual changes inthe response of the mechanical components over time, and to changes inthe response of the electrical components over time. They are alsosusceptible to accumulation of deposits on and within components andhousings. This drift is not significant where sensors are used for shortperiods, but may become significant where sensors are implanted and longterm use is desired.

SUMMARY OF THE INVENTION

It is an object of at least one embodiment of the present invention toprovide a sensor which goes some way to overcoming the abovedisadvantages or which will at least provide the health sector with auseful choice.

In a first aspect the present invention may broadly be said to be adevice comprising:

a housing enclosing a chamber and having at least one port thatcommunicates with the chamber,

a pressure sensor receiving fluid pressure from the chamber,

the chamber having a compliance that exhibits a marked change involumetric stiffness repeatably at a fixed pressure.

Preferably the device includes

a flexible wall portion forming part of a wall of the chamber,

a sealed cavity divided from the chamber by the flexible wall portion,such that an increasing pressure in the chamber causes increasingdeflection of the flexible wall portion,

the sealed cavity or housing being shaped such that as the pressure inthe chamber transitions through a first pressure part of the wallportion transitions from being in contact with a structure of the cavityor housing to being out of contact with the structure.

Preferably the flexible wall portion comprises a thin diaphragm andportions of the diaphragm can continue to deflect at pressures above andbelow the contact pressure.

Preferably the flexible wall portion is out of contact with thestructure at pressures below the first pressure, and in contact with thestructure at pressures above the first pressure.

Preferably there is no additional detector for determining contact ofthe flexible wall portion with the housing or cavity.

Preferably the pressure sensor is located in the housing.

Preferably the pressure sensor is within an enclosure sealed from thechamber by a flexible membrane, the enclosure being filled with anincompressible liquid.

Preferably at least one port includes connection for flexible tubing.

Preferably the housing includes an inlet port and an outlet port andboth inlet and outlet ports include connection for flexible tubing.

Preferably the device includes an interface from the pressure sensor toa controller.

Preferably the device includes a controller connected to the pressuresensor, an external communications interface connected with thecontroller and a power supply connected to supply power to thecontroller.

In another aspect the present invention may be said to broadly consistin an assembly comprising a device as hereinbefore described having afirst flexible tube extending from the inlet port and a second flexibletube extending from the outlet port.

In another aspect the present invention may be said to broadly consistin a hydrocephalus shunt comprising an assembly as hereinbeforedescribed and a shunt valve connected with the flexible tube extendingfrom the outlet port.

In another aspect the present invention may be said to broadly consistin a system including a device as hereinbefore described including aprocessor programmed to process data from the pressure sensor in acalibration method comprising identifying a pressure data point from thepressure sensor that corresponds with a time when the compliance of thechamber changes.

Preferably the program identifies the time when the compliance of thechamber changes by identifying a knee feature in a pressure data seriesrecorded over the duration of a calibration event.

Preferably the program identifies a knee feature according to distinctchanges in gradient of pressure over time.

Preferably the program sets a zero offset for use in relation to thepressure sensor based on the recorded output of the pressure at theidentified time.

Preferably the program sets a zero offset for use in relation to thepressure sensor based on the result of multiple instances of thecalibration method.

Preferably the program compares the results of multiple instances of thecalibration method and discards at least some of the results incalculating a new zero offset for the pressure sensor.

Preferably the processor may be any one or more of a microprocessor oran FPGA device array or complex instruction set computing (CISC)microprocessor, reduced instruction set computer (RISC) microprocessoror an ASIC device or digital signal processor or a CPU or a desktopcomputer or laptop or a smartphone or tablet or a cloud based processoror a cloud server.

In another aspect the present invention may be said to broadly consistin a recalibration method comprising the steps of occluding flexibletubes connected to ports of a housing of a pressure sensor, moving oneof the occlusions toward the housing

Preferably the occlusion is moved steadily toward the housing.

Preferably the diaphragm within the pressure sensor deflects incorrespondence to the increasing pressure within a chamber within thehousing, compensating for the loss of volume within a tube due to themovement of the occlusion.

Preferably the pressure sensor is configured to read the pressure in thechamber.

Preferably a discontinuity or change in the pressure rate is detectedand the discontinuity is used to recalibrate the value of the pressuresensor output based on the detected discontinuity value.

Preferably the discontinuity value is used as an offset or bias value torecalibrate the output of the pressure sensor.

This invention may also be said broadly to consist in the parts,elements and features referred to or indicated in the specification ofthe application, individually or collectively, and any or allcombinations of any two or more of said parts, elements or features, andwhere specific integers are mentioned herein which have knownequivalents in the art to which this invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

The term “comprising” as used in this specification means “consisting atleast in part of”. When interpreting each statement in thisspecification that includes the term “comprising”, features other thanthat or those prefaced by the term may also be present. Related termssuch as “comprise” and “comprises” are to be interpreted in the samemanner.

DESCRIPTION OF THE DRAWINGS

A pressure sensor will be described with reference to the accompanyingdrawings.

FIG. 1 is a cross section through a pressure sensor housing according toone sensor embodiment.

FIG. 2 is a perspective view of the pressure sensor housing of FIG. 1.

FIG. 3 is a schematic diagram illustrating the main parts of a deviceincluding housing, displacer, sensor, processor and communicationsinterface.

FIGS. 4a to 4d are charts outlining the operation of the processor intaking a sensor readings and in calibrating the sensor.

FIG. 5 is a graph plotting transducer output against time in a testsequence with a linearly increasing applied pressure.

FIG. 6 is a cross section of part of the transducer of FIG. 1illustrating, in an exaggerated form, deflections of the diaphragm atdifferent moments in the graph of FIG. 5.

FIG. 7 is a sketch illustrating location of the sensor device in use.

FIG. 8 is a diagram showing how a system including the sensor device maybe physically manipulated in performing a calibration.

FIG. 9 is a graph relating the pressure in the chamber of the housing tothe pressure in the housing when performing the calibration of FIG. 8.

FIGS. 10a to 10f are a series of graphs that illustrate processing ofsensor data to recognize a characteristic.

FIG. 11a shows a graph illustrating a knee pressure for a testingprotocol on a sensor.

FIG. 11b shows the touch down pressure values over 10 consecutivepressurizations to illustrate repeatability.

FIG. 11c shows an exemplary experimental set up for executing a testingprotocol on the pressure sensor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to pressure sensors, particularly forimplanting in the body. The sensor is intended for long term use, forexample for periods of at least months, or multiple years.

The sensor is intended for use in persons wishing the option of residingoutside the clinical environment, and who wish to go about their usualactivity.

For example the sensor may be used in persons having hydrocephalus. Thiscondition involves potential for excessive accumulation of cerebrospinalfluid within the ventricles of the brain. The disorder affects bothadults and children and is lifelong. This condition can be managed witha hydrocephalus shunt. However shunt failure due to blockage is common.The condition, or monitoring for correct operation of any implantedshunt, or both, could be better managed with reliable long termintracranial pressure monitoring.

Example devices are known in the prior art for this purpose. For exampleimplantable devices which include an absolute pressure sensor, processorand communication interface are described in U.S. Pat. No. 6,533,733with particular reference to FIGS. 3 and 4, and U.S. Pat. No. 6,248,080with particular reference to FIG. 1c , the relevant disclosures of whichare hereby incorporated in their entirety.

The present invention suggests adaptation of the sensor component ofsuch systems to provide a characteristic output curve (plotting sensoroutput against applied pressure) including a recognizable artefact thatreliably occurs at a pressure, and adaptation of the program executed bythe processor of the device to include a calibration function thatrecognizes the presence of this artefact.

For example, in a device constructed according to U.S. Pat. No.6,248,080 the pressure sensor module 20 would include features to createthe characteristic curve including recognizable artefact, and theprogram with recalibration function would be executed in microprocessor120. Alternatively the program with recalibration function could beexecuted in an external device (eg: device 500 in FIG. 3), incommunication with the implanted device, based on sensor data collectedby the implanted device. The external device could be any suitablecomputing device whether in close proximity to the implanted device orcommunicating over a network such as the Internet.

Referring to FIG. 1, one aspect of the invention comprises a sensorcomponent. The sensor component includes a housing 100. The housing 100includes an internal chamber 102. The housing 100 includes an inlet 104to the chamber 102 and an outlet 106 from the chamber 102. Outlet port106 is optional. An outlet port is not necessary for the pressuresensing or calibration, but may be useful if the device is used as partof a hydrocephalous shunt. The inlet and outlet may be essentiallyidentical, and the role of either port is not important. The housing maybe formed with any suitable connection or connector 108 at the inletport, the outlet port or both.

A pressure sensor 110 of the absolute type, for example a MEMS sensor,is located in the housing. The pressure sensor 110 is located in thehousing in pressure communication with the chamber. The pressure sensor110 is preferably located in a secondary space divided from the chamberby a membrane 112 in a cavity 114 defined by the housing and membrane.The cavity 114 is preferably filled with a substantially incompressibleliquid such as an oil. Pressure in the cavity is transferred to thesensor 110 through the oil 113.

A second cavity 116 is divided from the chamber 102 by a diaphragm 118.This cavity is sealed and contains a gas or vacuum. Preferably thiscavity contains a vacuum. A vacuum is temperature independent.Alternatively, the cavity may contain a gas at a known pressure. In thiscase the volumetric response will also be a function of temperature. Thetemperature can be measured by many MEMS pressure sensors such as theLPS22HBTR from ST Microelectronics. The diaphragm 118 may deflectaccording to the relative pressure between the cavity 116 and thechamber 102, thereby changing the volume of the chamber 102 according tothe fluid pressure in the chamber 102. For example, as the pressure inchamber 102 rises from a first pressure to a second pressure, thediaphragm deflects to progressively reduce the size of the cavity 116and increase the size of the chamber 102.

The cavity 116 and diaphragm 118 are formed so that there is adiscernible characteristic in the change in volume relative to change inpressure that occurs at a fixed relative pressure between the chamber102 and the cavity 116. In an embodiment this results from an abruptincrease in the effective stiffness of the diaphragm due to interactionbetween the diaphragm and an internal surface of the cavity 116. Forexample, in the illustrated embodiment, the surface of the cavityincludes an upstand 120. As the relative pressure increases, the centreportion of the diaphragm eventually rests on the upstand 120. Theannular portion of the diaphragm around the upstand 120 continues todeflect, but the effective stiffness of the diaphragm has been increasedby supporting the centre location.

According to other embodiments the cavity may include a second upstandthat will contact the annular portion of the diaphragm at a secondpressure higher than the pressure causing the centre of the diaphragm torest on the upstand 120. When the diaphragm is resting on both upstands,the diaphragm is still able to deflect but is now in a third state ofeffective stiffness.

According to further embodiments, the contact pressure of the diaphragmand the structure could be lower than the typical expected pressure, sothat the diaphragm and structure are usually in contact, and come out ofcontact by pressure reduction in the calibration process. Later anexample is given where a pinch process is performed to vary the pressurein the chamber, subsequently moving the pinch point in one directionwill increase the pressure in the chamber, while moving the pinch pointin the other direction will reduce the pressure in the chamber. In thisexample the pinch and roll process the roll would to increase the sizeof the occluded space.

According to further embodiments, the contact structure could beprovided in the chamber and contact the chamber side of the diaphragm,rather than the cavity side. In still further embodiments, contactingstructure could be provided on both sides, to contact at differentchamber pressures.

This is illustrated in FIG. 6, where the diaphragm is shown in a first,undeflected position 600 and a second position 602 (dashed line) when ithas deflected sufficiently to contact the upstand 120. The diaphragm isshown in a third condition 604 (dotted line), where the surroundingportion of the diaphragm has continued to deflect under furtherincreasing pressure.

The housing may be largely constructed from a stiff biocompatiblematerial. For example the housing may be constructed largely fromtitanium. The pressure sensor may include integrated electronics andcommunicates through the wall of the housing, so the housing may includea radio frequency transparent window, or the housing, or a portion ofthe housing, may be formed from a ceramic such asalumina/zirconia/borosilicate glass, or a polymer.

The secondary space in which the sensor is located is filled with apressure transmitting liquid, such as a silicone oil. This space isseparated from the chamber by a membrane sufficiently flexible tominimize the damping of transmitted pressure. For example this may be atitanium membrane with a thickness less than 25 micrometres.

The housing surrounding the cavity may also be constructed from a stiffbiocompatible material. For example this portion of the housing may beconstructed from titanium. The upstand may be integral with the housingor may be a separate component secured in place, for example byadhesive.

The diaphragm may be a titanium membrane. Preferably the housing is alsotitanium such that the cavity may be hermetically sealed by welding thediaphragm to the housing, for example by laser welding. The cavity maybe filled with an inert gas such as for example helium. Other inertgases may also be used to fill the cavity.

The housing ports may be formed with the housing, for example fromtitanium. The ports preferably are adapted for connection of tubing suchas medical grade silicone tubing.

As an implantable device the sensor component may be constructed to asize suitable to the particular purpose. For example for use in ahydrocephalus shunt, the sensor housing may be located subcutaneous,under the scalp of the patient, to one side of or to the back of thehead—such as indicated in FIG. 7. In this location the device may be asubstantial size, for example with an overall diameter of 10-15 mm andan overall thickness of 4-7 mm.

Where this component is included in a system, such as the hydrocephalusshunt of FIG. 7, the characteristic volumetric response to changingpressure in the chamber 102 can be used to recalibrate the pressuresensor 110. In such a system the sensor component 700 is located inseries in a drainage tube 702 that extends from the ventricles 704 ofthe brain, preferably above a shunt valve 706 which can open or closethe drainage tube according to a pressure condition in the brain.Accurate pressure readings in the shunt tube near the shunt valve arebeneficial to determining whether the shunt valve is operatingcorrectly.

For example, with reference to FIG. 8, a recalibration method mayinclude pinching or otherwise occluding flexible tubes connected to theports of the housing, as at locations 800 and 802. This defines anessentially sealed volume, including the chamber 804 and the portions ofconnected tube 806 and 808, containing a largely incompressible fluid(such as bodily liquids). One of the occlusions is then moved steadilytoward the housing, as illustrated by occlusion 807 and by arrow 810,thereby progressively reducing the amount of tube 808 included in thissealed volume. The diaphragm 812 deflects in sympathy with theincreasing pressure in the chamber 804, compensating for the loss ofvolume in the tube 808 by increasing the volume in the chamber. Thesensor 814 reads the pressure in the chamber 804 while this ishappening. A steady rate of movement of the pinch 802 leads to a steadyrate of compensation by deflection of the diaphragm 812. A plot of thepressure that is induced in the chamber 804 to force this deflectiondepends on the stiffness of the diaphragm. At a fixed pressure thisstiffness increases due to contact between the diaphragm and theupstand, and there is a substantial change in the rate at which thepressure in the chamber increases due to further reduction in volume ofthe tube. This is exemplified by the graph in FIG. 9, which plots thepressure in the chamber 804 against the volume in the cavity 820. A plotof pressure measured by the sensor 814 against time for such acalibration process will display a discernable discontinuity. An exampleplot is provided in FIG. 5. There is a discernible discontinuity in thegradient at location 500.

The output of the sensor can be processed to identify such adiscontinuity, and the sensor system can be recalibrated on theassumption that this discontinuity occurred at the known fixed pressure.

An implanted system may include processing and communication facilities.Such a system is exemplified in FIG. 3.

This implanted system includes a sensor component 11, with one or moreinternal sensors, including a pressure sensor, with the sensing chambercharacterized by a feature 300 (for example according to mannerdescribed above) to have a compliance to increasing pressure whichexhibits a discernible artefact. The sensors provide data to aprocessing device. The processing device may include a controller 13receiving data from the sensors via an interface 13. The controller 13can communicate with external devices via a communication interface 15.The system is powered from a power supply including power source 16 andpower conditioning 21. Such a processing device is described morecompletely in U.S. Pat. No. 6,533,733 which is hereby incorporated byreference.

In such a system there will have been an initial calibration of thesensor prior to implant. In addition, with the sensor componentdescribed above, there may be an initial calibration process todetermine the chamber pressure at which the chamber compliance exhibitsthe characteristic change. This pressure is stored as a referencepressure for future calibration events.

Processing of signals from the sensor to identify calibrationopportunities and to make recalibration steps may be completed by theprocessing device, by an external device, or by some combination ofdevices. Accordingly, the methods that will now be described could beimplemented in software of the implantable system, software of externaldevices in direct or indirect communication with the implantable system,or distributed across some combination of devices.

An example recalibration process is illustrated in FIGS. 4a to 4d . FIG.4a provides an overall process, and FIGS. 4b to 4c provide additionaldetail in relation to certain steps in the overall process. Therecalibration process is intended to be performed intermittently, withthe controller usually devoted to the activity of monitoring the sensoroutput.

The calibration process commences at step 400 with initialization. Wherethis process is being run in the implantable device this may be inresponse to a command received from an external device.

The process proceeds to step 402, to perform an individual calibrationevent. This is detailed in FIG. 4a , and proceeds in parallel withexternal influence in the form of artificial elevation of the pressure.Step 402 produces a pressure dataset covering the period of acalibration event. At step 404 the process analyses the pressure datasetto identify a characteristic feature and a sensed pressure (kneepressure) at which this feature occurred. This step is detailed in FIG.4b . The output of this step is the knee pressure at which a feature isdetermined to have occurred. Step 406 loops back to perform steps 402and 404 a number (“n”) times, so that actual recalibration can occurbased on multiple pressurization events to enhance accuracy andreliability. Once sufficient repetitions have been made the processproceeds to discard outliers at step 408. This is detailed in FIG. 4 d.

The process then proceeds to step 410 to decide whether to recalibrate.In particular the process compares the averaged knee pressure when theevents occurred against a reference value based on the presentcalibration of the sensor to determine an amount of drift since the lastrecalibration. This amount of drift is compared with a threshold. If thedrift is less than the threshold then the process proceeds to step 412and no revision is made to the reference value and the process ends. Ifthe drift is more than the threshold, then the process proceeds to step414. The threshold may be set at a level according to what would beclinically significant. For example for cerebral pressure monitoring thethreshold might be set as 2 mmHg or to some other value.

At step 414 the zero offset of the sensor is reset according to thedrift determined at step 410, or according to the difference between theknee pressure determined at step 408 and in initial knee pressure storedat an initial calibration of the device.

The process preferably also stores the determined drift at step 416 foruse in later analysis of sensor performance over time.

Step 402 is exemplified in greater detail with reference to FIG. 4b . Atstep 430 the implanted device communicates with an external device toestablish the start of a calibration event. The external devicepreferably includes a barometric pressure sensor, or can obtain datafrom a barometric pressure sensor.

At step 432 the process commences high frequency pressure sensing,expecting a calibration process that occurs over a period of seconds orminutes. This differs from the typical monitoring conducted by theimplanted device which may sample pressure on a much longer time scale,perhaps taking a sensor reading only each hour. The higher frequencypressure sensing may involve taking readings each second, or multipletimes per second such as 20 readings per second or more. The selectedfrequency will depend on the intended calibration technique. Thefrequency must provide sufficient data for subsequent analysis toreliably determine a knee in the pressure versus time curve.

At step 434 a user is prompted to start a pressurization event. Forexample, in the case of a pinch/roll calibration process, the user maypress closed the tube at either side of the pressure sense component,and then begin a roll of one finger toward the pressure sense component.

At steps 436 to 442 the process senses, calculates and stores pressuredata until a threshold pressure is reached. The threshold pressure issubstantially above the expected knee pressure, but below a pressurethat might damage the sense component or connections. In the loop, atstep 436 the pressure is read from the pressure sensor. A barometricpressure is read, preferably simultaneously, as at step 438, from anexternal barometric pressure sensor. The difference between the absolutesensed pressure and the barometric pressure is calculated and stored atstep 440, along with the time of the reading, or as part of a knowntimed sequence of readings. At step 442 the process determines whetherthe threshold has been met, returning to step 436 of the threshold hasnot been met.

Once the pressure threshold has been reached at step 442, the processprovides user feedback at step 444, preferably in the form of an audibleor visual alarm. This alerts the user to stop the process that isproviding the increasing pressure, as at step 446. This eventmeasurement process then terminates at step 448 by stopping pressuremeasurement.

An example process for determining the presence of an artefact in thepressure profile generated in a calibration event is set out in FIG. 4c. This commences at step 450 by retrieving the data stored for apressure calibration event, such as in the process of FIG. 4b . Thisdataset will include a time series of gage pressures.

At step 452 the process may crop data that is out of range. For examplethe process may crop one or other end of the dataset or both to includeentries that cover a time period when the pressure is continuouslyincreasing from a threshold below an expected knee pressure to athreshold above an expected knee pressure. This range is preferablychosen sufficiently large to definitely include the knee pressure, andto sufficient data either side of the knee pressure to establishderivatives of fitted curves of the data on either side of the expectknee pressure. This is illustrated on an example dataset in FIG. 10awhere data outside the region of interest defined by lower bound 900 andupper bound 902 is cropped, leaving a dataset covering only 17.5seconds. Again 10 a shows a plurality of data points about a line ofbest fit, the data points being shown as multiple dots or points on thegraph.

At step 454 the process filters the data set, for example using a lowpass filter, as a first smoothing stage. This is illustrated in FIG. 10bin relation to the example cropped dataset from FIG. 10a . In FIG. 10bthere are a plurality of dots or data points about the line thatrepresent the unfiltered data points. The line is a low band pass filterapplied to the data points i.e. like a line of best fit. The noise beingfiltered out produces a smoother curve.

At step 456 the process performs a smoothing operation, for example bycalculating a spline smoothing function to match the dataset. This isillustrated in FIG. 10c in relation to the filtered dataset of FIG. 10b. As seen in the graph of 10 c, the data points are shown as dots i.e.the low pass filter output. These data points are further smoothed byapplying a smoothing spline, as represented by the solid line.

At step 458 the process divides the data curve into sections using“knots”, and at step 460 fits a series of linear piecewise functionsacross the established knots. The knot placement step is illustrated inFIG. 10d . This can use one of many established tools for approximatinga given curve with a series of straight lines. The loop of steps 460,462, 470 and 472 seek to adjust knot placement to optimize the set oflinear fits to the smoothed data using a least squares approach, and tofix on a stable determination of a knee pressure. Ultimately thisoptimization should result in a knot being placed at or close to theknee pressure point, with the data immediately below the knee pressurepoint having a decidedly lower gradient than the data immediately abovethe knee pressure point. An example optimized knot sequence isillustrated by the vertical broken lines in FIG. 10d . In this case 10knots are placed.

At step 464 the gradients of each of the sequence of linear segments isdetermined. This data is illustrated in FIG. 10e , for the example dataof FIG. 10d . There are 9 data points, relating to the 9 line segmentsthat connected the 10 knots of FIG. 10d . At step 466 the changes inthis gradient data are determined. This first derivative of gradientdata is illustrated in FIG. 10f . There are 8 data points, indicatingthe 8 intermediate knots where line segments meet in FIG. 10d . In thisdata, the knot where the maximum change of gradient occurs is the knotdetermined to be associated with the pressure knee. In the example data,this is the third data point 920, which represents the thirdintermediate knot 922 in FIG. 10d , occurring at 6.5 s.

At step 468 the process records the pressure at the determined knotpoint. In the example dataset this gage pressure is 15 mmHg, asindicated at 924 in FIG. 10 d.

Once the loop finds an optimized set of knots with a stable kneepressure at step 472, this knee pressure is stored at step 474 as thedetermined knee pressure for this calibration event.

FIG. 4d illustrates in greater detail an example process for improvingthe reliability of recalibration by removing calibration data that maybe assumed to result from a faulty calibration process. The calibrationprocess is susceptible to imperfect technique, with the pressure beingraised in a jerky or non-smooth manner. This could result in instanceswhere the process determines the existence of a knee pressure that isnot the result of the correct operation of the sensor component.

In the process of FIG. 4d data from a sequence of calibration events isprocessed to determine an average knee pressure that excludes outliers.At step 480 the process retrieves knee pressure data for a series ofcalibration events. The number of events in this series is determined bythe loop at step 406 of FIG. 4 a.

At step 482 the process selects a subset of the values by determining agroup of close values, or selecting a set of values that are close tothe median value of the data set.

At step 484 the process determines the average value of the subset ofthe pressure data. The size of this dataset may depend on the size ofthe dataset. For example, this might be the closest 5 values, or theclosest half of the values.

At step 486 this is stored and returned as the averaged knee pressure,which the overall process of FIG. 4a will use to determine whether toset a new zero offset for the sensor, and if so, the value of the zerooffset.

This described process is an example only. Wide variation is possiblewithout departing from the intent of the invention, and while retainingone or more aspects of the process including: recognizing validcalibration data; processing data to determine characteristic featuresthat have been deliberately caused to exist at a repeatable pressurethrough the mechanical design of the sensor housing; checking thesedeterminations for sense by excluding outliers.

Variations on the sensor component are possible which would result inaccording variations of the process described. For example, the sensorcomponent may be designed such that the diaphragm exhibits multipledistinct changes in effective stiffness, such as by including more thanone interfering structure within the cavity, with these structuresengaging the diaphragm at different chamber pressures. Alternatively thesensor component may include multiple cavities divided from the chamberby independent diaphragms and interfering structures, each designed toexhibit the characteristic change in stiffness at a different chamberpressure. In this case the process would be modified to determinemultiple knee pressures, for example upper and lower knee pressures.This could be achieved for example by determining multiple peaks in the1st derivative of the inter-knot gradient.

In a further variation, the sensor component could include electronicdetermination of the diaphragm contacting the structure of the cavity,for example by capacitance or conductivity, independently of thepressure sensor. This would allow for more direct recalibration of thesensor at any time that the chamber pressure passes the contactpressure. However, this would preferably operate as an additionalrecalibration process alongside recalibration using the sensor pressurealone, as the electronic detection of contact could itself besusceptible to drift or other misfunction. In this case, a recalibrationprocess such as defined in FIGS. 4a to 4d could allow for recalibrationof the contact pressure of the diaphragm, recalibration of the pressuresensor, or both.

In a further variation, the calibration process including occluding theinlet and outlet tubes may be by means other than pinching a flexibletube, for example by valves. Further, the forced pressure increase byreducing volume may be by other than moving a pinch point on a flexibletube. For example a plunger or other squeezable chamber or sub-chambermay be provided.

In a further variation, the calibration process may include automation.For example pinch valves or other valves may be provided along withelectrical actuation, and the pressure increase (volume reduction) maybe provided by plunger or cam arrangement, also with electricalactuation.

Referring now to FIGS. 11a to 11c testing was conducted on a pressuresensor similar to the one described in this specification. The testingcomprised of a series of pressurization tests on a sensor having asimilar structure to the sensor described herein. The exemplary testsensor apparatus was similar in structure to the sensor described withreference to FIGS. 1 and 2, and also included compliant tubes similar toconnected tubes 806 and 808 described with reference to FIG. 8.

Based on the testing results it has been understood that the kneepressure (knee feature) i.e. the point at which there is a discernablediscontinuity in the pressure curve. In one testing protocol the inputpressure to the pressure sensor was produced using a syringe pump.However, this approach is difficult to replicate outside a laboratorysetting. At least one aim of the testing was to try and develop atesting protocol that can be used when the pressure sensor is in situ,i.e. in use and implanted within a user.

A second testing protocol that was used involved pressurization of thepressure sensor by using a person's fingers to depress the connectedtubes to the pressure sensor.

The pressure generated by the finger actuation can be regulated usingthe constricted flow path to reduce the rapid increase in pressure wherethe finger actuation occurs.

For the finger actuated testing protocol, the characteristic kneepressure can be detected by plotting the pressure values from thepressure sensor. A gradual pressurization without intermittentfluctuation was achieved. The experimental setup shown in FIG. 11ccomprises a pressure sensor. In the exemplary test set up 3 pressuresensors were pressurized and simultaneously recalibrated as part of thetesting in order to show the recalibration process can be repeated. Therecalibration method used was the finger pinching method, as describedherein. The testing protocol also comprises detecting the knee pressureover 10 pressurization cycles. FIG. 11c shows an exemplary test set upthat was used to test the pressure sensor and show that recalibrationcan be repeatedly achieved. As seen in FIG. 11c the test set upcomprises a pressure source 1110 which may be a syringe or anotherpressure source. The test set up also includes a pressure switch 1112and the pressure sensor 1114. The pressure switch 1112 is in fluidcommunication with the pressure sensor and the pressure source 1110 isin fluid communication with the pressure switch 1112.

FIG. 11a shows the knee pressure that is detected from one sensor over10 consecutive pressurization cycles. As shown in FIG. 11a there is adiscernible knee as indicated by the red circle 1102 at a time ofapproximately 95 seconds after the pressure calibration test began. Asshown in FIG. 11a , the lighter line represents the piecewise linearfit. The dashed lines represent the optimal knot locations i.e. pressureknot locations. The dark line (made of a plurality of dots adjacent eachother) are the unfiltered pressure data. The line of best fit passesthrough a majority of the pressure data to create a best fit line.

FIG. 11b shows the knee pressure points detected from one sensor over 10consecutive pressurization cycles i.e. the touchdown pressures. Thetouchdown pressure is the pressure value where the membrane contacts thefirst upstand. The testing protocol using a regulator provides animproved testing protocol that is repeatable. The testing protocol alsoshows that the sensor device can be recalibrated to an accuracy ofapproximately plus or minus approximately 0.1 mmHg. Clinically this isvery useful since the finger pinching based recalibration method can berepeatedly used.

Embodiments of the present invention provide for an implantable pressuresensor that can be recalibrated in situ, non-invasively. This extendsthe useful lifetime of the sensor by allowing compensation for long termdrift. The compensation is determined in relation to the performance ofa mechanical system which is expected to exhibit minimal drift andvariance over time.

While the pressure sensor component, and system, is particularlydescribed in relation to an implantable sensor for use in the medicalsphere, it could also be adapted for other applications requiringsensors in locations that are hard to access. A system with automationof the calibration process would be particularly applicable to such anapplication where necessary miniaturization may prove less challenging.

The invention claimed is:
 1. A device comprising: a housing enclosing achamber and having at least one port that communicates with the chamber,a pressure sensor receiving fluid pressure from the chamber, the chamberhaving compliance that exhibits a marked change in volumetric stiffnessrepeatably at a fixed pressure, wherein the pressure sensor is locatedin the housing and within an enclosure and sealed from the chamber, theenclosure being filed with an incompressible liquid and the pressure inthe enclosure is transferred to the pressure sensor through theincompressible liquid, wherein the device further comprises: a flexiblewall portion forming part of a wall of the chamber, a sealed cavitydivided from the chamber by the flexible wall portion, such that anincreasing pressure in the chamber causes increasing deflection of theflexible wall portion, the sealed cavity comprising a structure in aform of at least one upstand such that as the pressure in the chambertransitions through a first pressure, part of the wall portiontransitions from being in contact with the structure of the cavity tobeing out of contact with the structure.
 2. The device as claimed inclaim 1, wherein the flexible wall portion forms part of the wall of thechamber and the sealed cavity is divided from the chamber by theflexible wall portion such that the pressure in the chamber rises fromthe first pressure to a second pressure, the size of the sealed cavityis reduced and the size of the chamber is increased.
 3. The device asclaimed in claim 2, wherein the flexible wall portion comprises adiaphragm and portions of the diaphragm permits continued deflection atpressures above and below a contact pressure.
 4. The device as claimedin claim 3, wherein the flexible wall portion is out of contact with thestructure at pressures below the first pressure, and in contact with thestructure at pressures above the first pressure.
 5. The device asclaimed in claim 2, wherein there is no additional detector fordetermining contact of the flexible wall portion with the housing orcavity.
 6. The device as claimed in claim 1, wherein the incompressibleliquid is oil.
 7. The device as claimed in claim 6, wherein theenclosure is sealed from the chamber by a flexible membrane.
 8. Thedevice as claimed in claim 1, wherein the at least one port includesconnection for flexible tubing.
 9. The device as claimed in claim 8,wherein the housing includes an inlet port and an outlet port and bothinlet and outlet ports include connection for flexible tubing.
 10. Anassembly comprising the device according to claim wherein a firstflexible tube extends from the inlet port and a second flexible tubeextend from the outlet port.
 11. A hydrocephalus shunt comprising theassembly as claimed in claim 10 and a shunt valve connected with theflexible tube extending from the outlet port.
 12. The device as claimedin claim 1, including an interface from the pressure sensor to acontroller.
 13. The device as claimed in claim 12, including acontroller connected to the pressure sensor, an external communicationsinterface connected with the controller and a power supply connected tosupply power to the controller.
 14. A system including the device asclaimed in claim 1 including a processor programmed to process data fromthe pressure sensor in a calibration method comprising identifying apressure data point from the pressure sensor that corresponds with atime when the compliance of the chamber changes.
 15. The system asclaimed in claim 14, wherein the program identifies the time when thecompliance of the chamber changes by identifying a knee feature in apressure data series recorded over the duration of a calibration event.16. The system as claimed in claim 15, wherein the program identifies aknee feature according to distinct changes in gradient of pressure overtime.
 17. The system as claimed in claim 14, wherein the program sets azero offset for use in relation to the pressure sensor based on therecorded output of the pressure at the identified time.
 18. The systemas claimed in claim 17, wherein the program sets a zero offset for usein relation to the pressure sensor based on the result of multipleinstances of the calibration method.
 19. The system as claimed in claim18, wherein the program compares the results of multiple instances ofthe calibration method and discards at least some of the results incalculating a new zero offset for the pressure sensor.